Rule Request: Preprocessor flag statements inequality check

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

For code that is controlled via preprocessor flags (#if DEBUG etc.), an error/warning should appear if all statements embedded in flag check are equal.

1. Why should this rule be added?

Such a check can prevent unexpected runtime behavior if the app doesn't have unit and/or UI tests implemented.

2. Provide several examples of what would and wouldn't trigger violations.

Triggering examples:

static let shouldShowDebugMenu: Bool = {
#if DEBUG
  return true
#else
  return true ↓
#endif
}()

var environment: String {
#if DEBUG
  return "Debug"
#elseif BETA
  return "Debug" ↓
#else 
  return "Debug" ↓
#endif
}

var isPrerelease: Bool {
#if DEBUG || BETA
  return true
#else
  return true ↓
#endif
}

let viewController: UIViewController
#if MOCKING
viewController = FooViewController()
#else
viewController = FooViewController() ↓
#endif 

Non-triggering examples:

static let shouldShowDebugMenu: Bool = {
#if DEBUG
  return true
#else
  return false
#endif
}()

var environment: String {
#if DEBUG
  return "Debug"
#elseif BETA
  return "Beta"
#else 
  return "Production"
#endif
}

var isPrerelease: Bool {
#if DEBUG || BETA
  return true
#else
  return false
#endif
}

let viewController: UIViewController
#if MOCKING
viewController = FooViewController()
#else
viewController = BarViewController()
#endif 

3. Should the rule be configurable, if so what parameters should be configurable?

I can't imagine any options, apart from warning/error level.

4. Should the rule be opt-in or enabled by default? Why?

Preprocessor flag checks are hard to catch unless there are unit or UI tests in the project running on different environments. I would say this should be enabled by default since it prevents runtime issues, potentially affecting production environments.

No files, tests, or entry points are named. Start by locating SwiftLint's existing rule implementations and preprocessor-related tests, then compare them with the triggering and non-triggering examples in this issue. Done means the rule detects equivalent branches, handles the shown conditional forms, and has coverage for both triggering and non-triggering cases.